By DE-A-34 35 128 a filter assembly for installtion in a flow conduit is known, which has a connecting portion with two aligned connecting sockets for installation in a flow conduit. These two connecting sockets form the inlet and the outlet of the filter assembly. The connecting portion comprises a lateral tubular outer projection. An inner projection is provided coaxially in the outer projection. The outer projection forms an annulus around the inner projection. This annulus is connected to the inlet. The annulus is separated from the outlet by a partition. The outlet is connected to the interior of the inner portion. A filter cup is fixed with its edge to the outer projection. A cylindrical filter, a "filter candle", is located on the inner projection. The filter, at its end remote from the connecting portion, is held in a guidance, which is provided at the bottom of the filter cup. The filter cup has an aperture at its bottom. A pressure reducer is inserted into the filter assembly coaxially to the filter. The inner projection has a central aperture, into which a valve seat of the pressure reducer is sealingly inserted.
In this prior art filter assembly the water flows from the inlet through the annulus of the connecting portion into the space between filter cup and filter. Then it flows through the filter around the housing of the pressure reducer and through the valve of the pressure reducer into the inner projection and to the outlet. The outlet pressure is controlled by the pressure reducer.
This prior art filter assembly does not permit any backwashing of the filter. Therefore, it is necessary to exchange or clean the filter in certain time intervals. To this end, the filter assembly has to be detached. For that purpose the water supply has to be shut off, such that the supply is temporarily interrupted.
Filter assemblies are known, which permit backwashing. The filter can be cleaned without the need of dismantling the filter assembly or of interrupting the water supply.
In a prior art filter assembly of this type, the water is guided from the inlet into the interior of a cylindrical filter. Then it flows from the inside outwards through the filter and then flows on the outside of the filter through the annulus between filter and housing to the outlet. In the interior of the filter a backwashing device is axially movably guided. This backwashing device has limited suction openings, which engage the inner surface of the filter on two axially spaced ring surfaces. These suction openings communicate with an outlet valve through passages. The outlet valve is opened by hand to obtain cleansing of the filter by backwashing. Then the backwashing device is axially shifted, also by hand, such that the suction openings move over the whole inner surface of the filter. The use of limited narrow suction openings offers the advantage, that a high flow rate results in backwashing direction through the filter, without the total amount of the water amount flowing through the outlet valve to a drain becoming inadmissably large. The pressure difference between water conduit pressure and atmospheric pressure in the drain essentially is maintained across the suction openings. The high flow rate ensures, that the impurities retained by the filter are washed out with good efficiency during the backwashing process.
The prior art filter assembly requires manual operation both of the outlet valve and of the means for axially shifting the suction device. Because the suction tube is inside the filter, it is not possible to provide a pressure reducer at that location to save space, as this is done in the filter assembly of DE-A-34 35 128 mentioned first.
Further, a filter assembly permitting backwashing is known (company brochure "Ruckspulbare Hauswasser Station HS 10" of Honeywell Braukmann), wherein two cylindrical filters are arranged in a filter cup, which is attached to a connecting piece. The filter cup is similarly constructed as in the prior art filter assembly mentioned first. It has aligned connecting sockets as inlet and outlet and an outer and and inner tubular lateral projection. The filter cup is connected to the outer projection, which is connected to the inlet. The first cylindrical filter is fixedly connected to the inner projection. The second cylindrical filter is arranged coaxial to the first filter and is axially movable. The second cylindrical filter is closed by a plate at its end face remote from the first filter. A spring urges the second filter in the direction towards the connecting piece. The displacement of this second filter against the action of the spring is limited by a stop. An outlet valve is provided at the bottom of the filter cup.
In normal operation, the water flows, as in the filter assembly already described above, from the inlet through the annulus between filter cup and filters from outside to the inside through both of the filters and then inside the filter to the inner projection and to the outlet. The outlet valve is opened for backwashing. Thereby the second cylindrical filter is shifted a little towards the bottom of the filter cup due to the water pressure acting on the plate. Thereby the annulus formed around the second filter is separated from the annulus around the first filter, which annulus is connected to the inlet, due to an edge of the filter cup projecting inwards and an edge at the upper end of the second filter projecting outwards. Now the water flows from the inlet only through the first filter. However, in the second filter, filtered water flows in an opposite direction, from the inside-out, to the outlet valve and to a drain.
In this prior art arrangement, only part of the total filter surface, namely the second filter, is cleaned by backwashing. Water flows simultaneously through the whole surface of the second filter. If in such an arrangement the total water is limited, the flow rate of the water during the backwashing becomes rather low. Therefore, the cleaning effect of the backwashing is limited.
DE-A-37 14 660 shows a filter assembly to be installed a flow conduit. The filter assembly comprises a cylindrical filter and a suction device, which engages the outer surface at the inlet side with limited suction openings. The suction device can be connected to a drain through an outlet valve. A displacement member is provided to move the suction device over the surface of the filter. This displacement member is exposed, on one side, by pressure of the inlet and, on the other side, limits a space, which is connected to the inlet through the suction tube acting as restrictor and is closed by the outlet valve. A pressure difference across the displacement member and the suction device results when the outlet valve is opened. Thereby the displacement member is displaced relative to the housing fixed filter. The suction device is attached to the displacement member, such that the suction device is moved thereby over the surface of the filter. The pressure difference at the narrow suction opening of the displacement member causes a strong flow through the filter and into the suction opening, such that the filter is washed in the reverse flow. A pressure reducer is centrally located in the filter and projects with a tubular housing into the filter housing.
The prior art construction is rather expensive. The filter is held at the outlet socket and at the housing of the pressure reducer through supporting rings. Then, in addition, the displacement member is necessary, which is slidably guided and from which the suction passages project upwards around the filter and support the suction openings at their upper end. The displacement of the displacement member is limited, because the displacement member, in its position of res, has to be below the lower supporting ring of the filter. Therefore two sets of different length suction passages with associated suction openings are required. The suction passages are mounted on the displacement member at one end only and engage the filter with their free ends with the suction openings. Thereby no safe engagement is ensured.